In view of increased demand for four-wheel drive vehicles, many power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In some vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode.
A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with a torque transfer mechanism having a transfer clutch and a power-operated clutch actuator. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between rotary components of the primary and secondary drivelines and which is actuated by the power-operated clutch actuator in response to control signals sent from a controller. The control signals are typically based on current operating characteristics of the vehicle (i.e., vehicle speed, interaxle speed difference, acceleration, steering angle, etc.) as detected by various sensors. Thus, such torque transfer mechanisms can utilize adaptive control schemes for automatically controlling adaptive torque distribution during all types of driving and road conditions.
Currently, many transfer cases are equipped with a torque transfer mechanism having a power-operated clutch actuator that can automatically regulate the amount of drive torque transferred across the transfer clutch to the secondary output shaft as a function of the electric control signal applied thereto. In some applications, the torque transfer mechanism employs an electromagnetic clutch as the power-operated actuator. For example, U.S. Pat. No. 5,407,024 discloses a electromagnetic coil that is incrementally activated to control movement of a ball-ramp operator for applying a clutch engagement force on a multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a torque transfer mechanism equipped with an electromagnetic clutch actuator for directly controlling actuation of the multi-plate clutch pack assembly.
As an alternative, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a torque transfer mechanism equipped with an electric motor that controls rotation of a cam plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a torque transfer mechanism which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging a multi-plate clutch assembly. Finally, U.S. Pat. No. 4,895,236 discloses a transfer case equipped with a torque transfer mechanism having an electric motor driving a reduction gearset for controlling movement of a ball screw operator which, in turn, applies the clutch engagement force to the multi-plate transfer clutch.
To further enhance the tractive and stability control characteristics of four-wheel drive vehicles, it is known to equip such vehicles with brake-based electronic stability control systems and/or traction distributing drive axle assemblies. Typically, such drive axle assemblies include a drive mechanism that is operable for adaptively regulating the side-to-side (i.e., left-right) torque and speed characteristics between a pair of driven wheels. In some instances, a pair of modulatable transfer clutches are used to provide this side-to-side control as is disclosed, for example, in U.S. Pat. Nos. 6,378,677 and 5,699,888. As an alternative, a hydraulically-operated traction distribution axle assembly is shown in U.S. Pat. No. 6,520,880. Additional traction distributing axle assemblies are disclosed in U.S. Pat. Nos. 5,370,588 and 6,213,241.
While many power-operated clutch control systems similar to those described above are currently used in four-wheel drive vehicles, a need exists to advance this technology and address recognized system limitations. For example, the size, weight and electrical power requirements of the electromagnetic coil or the electric motors needed to provide the described clutch engagement loads may make such system cost prohibitive in some motor vehicle applications.